1. Field of the Invention
The present invention relates generally to storage-stable, citrus-flavored compositions and to methods for their preparation. In particular, the invention relates to storage-stable foods and beverages containing citral as the flavoring agent and a plant extract as the stabilizing agent.
2. Description of the State of the Art
Citral (shown as the isomer neral (A) in FIG. 1) is the key flavor component in citrus fruits (e.g., lemon, lime, grapefruit and orange), fresh citrus juices, and lemon grass. Unfortunately, citral is very unstable and rapidly decomposes through a series of cyclization and oxidation reactions, particularly in the presence of an acid. (Baines, D., et al., Tetrahedron, 26:4901-4913 (1970); Slater, C. A. and Watkins, W. T., Journal of Science and Food Agriculture, 15:657-664 (1964); Clark, Jr., B. C., et al., Tetrahedron, 33:2187-2191 (1977); and Clark, Jr., B. C. and Chamblee, T. S., Off-Flavors in Foods and Beverages (Charalambous, G., editor), pp. 229-285 (1992).) The lower the pH of the citrus solution, the higher the rate of cyclization (Freeburg, E. J., et al., Perfumer and Flavorist, 19:23-32 (1994)). In addition to reducing the fresh citrus flavor, these oxidation and cyclization reactions produce potent flavor and odorant compounds, which significantly decrease the shelf life of citrus-flavored products.
FIG. 1 is a schematic diagram showing the cyclization and oxidation reactions in the citral degradation pathway. The major reaction products of citral degradation, under both mild and astringent aqueous conditions, are cyclic alcohols (compounds B and C), which have a bland, mild fruity taste. Alcohol production continues until the citral is depleted. (Clark and Chamblee, supra.) In the presence of oxygen, compounds B and C oxidize to p-cymen-8-ol (compound D), which dehydrates to p-.alpha.-dimethylstyrene (compound E). Further oxidation of p-.alpha.-dimethylstyrene produces p-methylacetophenone (compound F) and p-cresal (compound G). (Schieberle, P., et al., Z. Lebensm. Unters. Forsch., 187:35-39 (1988).) All four of these oxidation products (compounds D, E, F and G) are believed to contribute to off-flavor development, which limits the shelf life of citrus-flavored foods and beverages. Although each of these breakdown products are thought to affect the quality of aged citrus products, the relative contributions of compounds D, E, F and G to off-flavor development depend upon the taste thresholds of these compounds. For example, the taste threshold of p-cymen-8-ol (compound D) is high compared to other oxidation products, particularly p-methylacetophenone (compound F). Thus, considering its threshold, p-cymen-8-ol does not contribute as significantly to the off-flavor of deteriorated citrus products as do other citral oxidative degradation products.
Methods for stabilizing foods and flavor particles, including aromatic compounds such as fruit flavors and citrus oils, have been proposed. U.S. Pat. No. 5,603,952 to Soper describes a method for forming microencapsulated food and flavor particles. Soper's method involves encapsulation with fish gelatin (complex coacervation) to protect the flavorant from degradation.
Freeburg et al., supra, refers to a method for stabilizing lemon-flavored beverages by making a citralless lemon solution. The referenced method removes citral from the flavor system, thereby eliminating off-flavor formation due to citral degradation. This citralless flavor system lacks the fresh lemon/fruity characteristics associated with citral.
Other researchers have investigated the effects of various antioxidants on citral degradation, and specifically the formation of oxidative degradation products (compounds D, E, F and G). Kimura et al. report that none of the free-radical terminators (antioxidants) they tested (i.e., butylated hydroxytoluene (BHT), butylated hydroxyanisole (BHA), propyl gallate, d,l-.alpha.-tocopherol, nordihydroguaiaretic acid and n-tritriacontane-16,18-dione, isolated from the leaf wax of the Eucalyptus tree) inhibited the formation of these citral oxidative degradation products in an aqueous citral solution. (Kimura, K., et al., Journal of Agricultural and Food Chemistry, 31:801-804 (1983); and Kimura, K., et al., Agricultural and Biological Chemistry, 47:1661-1663 (1983).) Because these antioxidants failed to prevent formation of oxidative products, Kimura et al. concluded that citral degradation can proceed in the absence of oxygen. However, Peacock and Kuneman report that isoascorbic acid, an oxygen scavenger, inhibits the formation of p-cymen-8-ol (compound D) and p-.alpha.-dimethylstyrene (compound E). (Peacock, V. E. and Kuneman, D. W., Journal of Agricultural and Food Chemistry, 33:330-335 (1985).)
The rate of citral degradation is accelerated in foods and beverages packaged in gas permeable materials, such as polyethylene. Such materials allow oxygen to diffuse through the packaging, thereby facilitating the oxidation reactions. One product of these oxidation reactions, p-methylacetophenone (compound F), has a strong, bitter almond-like flavor and odor. Because the taste and odor thresholds of p-methylacetophenone are low relative to the other flavor components, this compound is a major contributor to the undesirable off-flavor and odor associated with aged citrus-flavored food products. (Freeburg et al., supra.) None of the above-referenced antioxidant compounds are effective in preventing formation of this potent odorant.
A need therefore exists for a method for improving the stability and shelf life of citrus-flavored compositions, particularly citral-flavored foods and beverages. More specifically, a need exists for a stabilizing agent which effectively inhibits the formation of the potent flavorant and odorant p-methylacetophenone.